Free electricity

I got solar about 6 months ago. At that time I did not have a battery or an export tariff. In fact it took about 5 months to get any sort of export arrangement because of delays in paperwork.

So back then... (June)

This meant I was getting free electricity. When the sun was shining I could decide to use something, like the hot tub, or tumble dryer, and use that sunshine, and the marginal cost of doing so was zero. I neither gained, nor lost, any money by doing so.

Of course there are caveats, if I was to use too many things or use them on a cloudy day when there was not the solar, I would be paying for the extra usage. So only free electricity up to a point. But still, it was guilt free using the hot tub, etc.

Payback time (August)

Finally I got an export tariff, albeit only 7½p/unit, which has yet to actually pay due to So Energy dragging their heels. But in principle, at that point, my excess solar was paying me 7½p/unit. This meant I no longer had free electricity. If I used the hot tub, even when sunny, it was costing me 7½p/unit. But that is really cheap so almost guilt free.

Battery time (September)

Finally I got the battery install, and that changes things a lot.

Because the battery can charge up on the excess solar and then be used to power the house, whist the battery is not full, and whilst we are not making excess solar more than 5kW, the battery takes the power.

In the morning, if there is enough sun to fill the battery as above, all of that extra sunshine can be used any time during the day and it will just delay the point the battery is full. So costs me the export (5p) that I am reducing. But once passed that, at the point the battery does not get full, any usage will simple mean the battery runs out faster and so will mean power from the grid, so cost me normal rate (currently 24p).

Of course, whether the usage in the morning is nearly free electricity (5p) or not (24p) depends on how much sunshine happens later in the day. It is Schrödinger's free electricity - you have to wait to open the box.

Some symmetry (October)

I am now on a new tariff with Octopus, the idea is they control the battery now, and use it to charge and feed in to the grid as needed. As they control it, they have taken the sensible approach of making the electricity a symmetric pricing, I pay 24p/unit but I get 24p/unit for export, so end up paying for my net usage at 24p/unit.

This means no more free electricity. Any usage at any time will cost me 24p/unit for that usage. Similarly any sunshine (up to my 6kW export limit after any battery charging) is 24p/unit benefit.

This is, at least, simple, and does not depend on how sunny the afternoon is - the cost is known, but does mean no more free electricity. All usage costs.

Battery feeding the grid in evening

Battery charging at night

Next step (when hell freezes over)

The next step gets fun. I am getting a second battery (which are like hen's teeth). When I get it I will be able to change to a night time EV tariff, charging it (and heating hot tub) in the night at something daft like 7½p/unit. Then I use battery and solar to run the house during the day.

This means all my electricity will be cheap, 7½p/unit, well, up to a limit. If I use too much, and then if there is not enough sunshine, I end up using during the day as the battery will run out. At that point I am paying more like 40p/unit.

So, once again Schrödinger's free electricity - I won't know what extra usage will cost until end of the day depending on how sunny it was, but as the battery plus cloudy solar should cover most of our normal usage, it should always be the 7½p rate. The only caveat is I may want to shut down hot tub until next day if no sunshine, and I may want to discourage tumble dryer on cloudy days.

Every little helps

Even though some of these options mean no free electricity, all of these steps reduce my overall bill, a lot. The solar now means I am paid for electricity I make at a sane rate, which is excellent. Indeed, some options could mean exporting extra battery storage at the end of the day to get paid more than it cost to get the charge in the morning. It could make some days negative total price.

Nothing is free

Don't forget, it is only free if you also forget the huge installation cost. Start here.


  1. You're never "paying" the 5p for electricity, it's a loss of profit. And all the profit you're making from those 5p exports should be discounted against the cost of your electricity at other times.

    1. "paying" and "loss of profit" have EXACTLY the same impact on my bank balance, so yes I am...

  2. What happens to your daily charge?
    I haven't heard of anyone factoring that into the cost but, to my mind, it's value is the opportunity cost of running out of battery. If you knew you weren't going to run out of battery you could be entirely off-grid and have guaranteed monthly bills of zero.
    I have an interest in an analysis of that cost because my utility bills are almost entirely dominated by the standing charges. (I have a small place, live on my own and don't use much power).


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